So adding to the $PHP_VER array worked after a couple attempts. For anyone else wanting to test the new version remember to use '5.5.0' because '5.5' won't work.
The only problems I had were it kept failing when I had PHP Mail Header Patch selected, which was there by default. Tried again and it failed, this time it was an issue with APC. Deselecting APC as well I finally got it to work. I hear Zend Optimizer+ is just as good in 5.5, so I might try that and see how it goes.